The present invention concerns direct current biased superconducting quantum interference devices (D.C. SQUIDs). The D.C. SQUID is an extremely sensitive magnetic flux sensor. When biased with a constant current the voltage across the SQUID is a periodic function of the applied magnetic flux, with the period being equal to one flux quantum (.PHI..sub.o =2.07.times.10.sup.-15 Weber). The prior art teaches that the two physical points at which current should be injected into the D.C. SQUID device, in order to accomplish the biasing thereof, should be symmetrical relative to the physical locations of the two Josephson junctions within the D.C. SQUID device. So biased, the D.C. SQUID has a zero magnetic flux input operating point at a minimum output (voltage or current). As magnetic flux either increases or decreases (i.e. is coupled at non-zero magnitude to the D.C. SQUID in one direction or the other) about the zero magnetic flux operating point, the output (voltage or current) increases about the minimum output (voltage or current) produced at zero magnetic flux input, such output (voltage or current) assuming a periodic function with applied magnetic flux.
In the prior art mode of operating such a symmetrically biased D.C. SQUID, a modulated signal is detected by room temperature electronics and fed back to the SQUID input to maintain an operating point near the minimum in the periodic output (voltage or current) versus input magnetic flux characteristic. Slew rate, dynamic range and system noise are limited by the room temperature electronics. All these limitations are undesirable in maximizing the time responsiveness, dynamic range, and sensitivity of the D.C. SQUID to magnetic flux. The present invention will later be seen to involve (1) the construction of asymmetrically biased D.C. SQUIDS operative to produce a linear (voltage or current) output about zero input magnetic flux, and (2) the construction of amplifiers from such asymmetric SQUIDS.
Useful in understanding the present invention are a number of parameters used to characterize SQUID behavior. The principal electrical model of a SQUID has been the RSJ model since its publication by W. C. Stewart in Appl. Phys. Lett. 12, 277 (1968) and by D. E. McCumber in J. Appl. Phys. 39, 3113 (1968).
The RSJ model treats each junction as an ideal Josephson element, I.sub.o, in parallel with a resistive shunt, r, and a capacitance, C. The SQUID loop has an inductance, L. The following formulas (1) through (4) list a number of criteria these parameters must satisfy for the SQUID to function properly. Beyond these criteria, the primary figure of merit is SQUID noise, as it determines the detection limit of the SQUID.
(1) (2 .pi./.PHI..sub.o) I.sub.o rrC.ltoreq.1 (for Non-Hysterectic Josephson junctions) PA1 (2) (I.sub.o .PHI..sub.o /2.pi.)&gt;&gt;kTI.sub.o &gt;0.2 .mu.a (in order that the Josephson junction stay locked) PA1 (3) (.PHI..sub.o.sup.2 /L)&gt;4kTL&lt;2nH (in order that the SQUID stay locked) PA1 (4) (2LI.sub.o /.PHI..sub.o).perspectiveto.1 (so that coupled energy is evenly distributed between loop and Josephson junction)
The electrical model of the present invention will be seen to additionally recognize a parasitic bridge capacitance (k) between the two Josephson junctions. The present invention will then teach the use, and the value selection, of a resistance (R) in parallel, or shunt, relationship to such a parasitic capacitance (k). An electrical model of a SQUID shown by C. D. Tesche in the Journal of Low Temperature Physics volume 47 page 385 (1981) shows the bridge capacitance (k) and a series resistance (R), which series resistance (R) is possibly not of discrete implementation but rather only part of Tesche's electrical model for a SQUID. Whether or not Tesche's resistance (R) is of actual discrete implementation or, as seems probable, only part of his model, the present invention will teach the implementation of a discrete real resistance (R) of a specific calculated value in parallel, or shunt relationship with a parasitic capacitance (k) of the SQUID. This parallel, or shunt, resistive R is not merely part of the electrical model of the SQUID, but also a discrete physically realized entity in the preferred embodiment thin film technology in which the SQUID of the present invention is preferably implemented.
Prior art effort to build an amplifier from SQUIDS is revealed in the paper "An Integrated DC SQUID Cascade" by A. Davidson presented at the 1982 Applied Superconductivity Conference. In the words of Davidson's abstract to his conference presentation:
"We show experimentally that it is feasible to couple all the available current from one thin film dc SQUID to another on the same chip. This cascade may be one way to solve the read-out problem for the highly sensitive tunnel junction dc SQUID's that have been produced by the Josephson computer technology at IBM (and elsewhere), while preserving a large bandwidth (the order of 1 GHz). The technique involves coupling the shunt resistors of the first SQUID in the cascade to a transmission line, a filter, and the input inductance of the second SQUID. Hence the resistive shunts for the first SQUID are not directly across each junction, but are completed by an extended circuit that would ordinarily disrupt proper SQUID action. We show nonetheless that the voltage versus flux characteristic of the first stage is not affected by the coupling; that all the current is transferred; and that the signal response time is less than four nanoseconds. Parasitic phase locking between the two SQUID's has so far prevented a demonstration of low noise read-out."
The present invention is not based on coupling the shunt resistors of a SQUID.